Typically, a horizontal-axis wind turbine includes a tower extending vertically, a nacelle which is attached to the upper end portion of the tower such that the nacelle is horizontally rotatable, and a rotor which is rotatably mounted to the nacelle. The rotor includes a hub provided at a center portion thereof, and plural blades which are attached to the hub and are rotatable within a substantially vertical operation plane. The horizontal-axis wind turbine is classified into an upwind wind turbine including blades directed upwind during power generation and a downwind wind turbine including blades directed downwind during power generation.
A wind load applied to a wind turbine while a strong wind is blowing, for example, in a typhoon, is a main factor for determining strength of a structure of the wind turbine. A ratio of the wind load applied to the blades to the wind load applied to the wind turbine is particularly high. Therefore, by reducing the wind load applied to the blades, a wind pressure resistance of the wind turbine can be improved without increasing a cost to ensure strength of the structure. Various techniques have been proposed to reduce the wind load applied to the blades.
Patent document 1 discloses an upwind wind turbine. This wind turbine is placed in an upwind position in which all blades are directed upwind during power generation. When an anemometer detects a wind speed which is not lower than a cut-out wind speed, all of the blades are switched to be feathered and the rotor stops. Then, the nacelle yaws in a range of 75 to 110 degrees and all of the feathered blades are switched to be reversed. Then, the nacelle yaws and this wind turbine is placed in a downwind position in which all of the blades are directed downwind.
Patent document 2 discloses a downwind wind turbine. This wind turbine includes a blade tilting mechanism for adjusting tilting angles of the blades to tilt the blades in a downwind direction to reduce a blade passage area while a strong wind is blowing. This makes it possible to reduce the wind load applied to the blades without stopping a power generation operation of the wind turbine.
Patent document 3 discloses a downwind wind turbine. This wind turbine includes a blade tilting mechanism for adjusting tilting angles of blades to change tilting angles of the blades according to an average wind speed detected by an anemometer. This makes it possible to reduce the wind load applied to the blades without stopping a power generation operation of the wind turbine and to adjust a blade passage area to increase an output power of the wind turbine.    Patent document 1: Japanese Laid-Open Patent Application Publication No. 2007-064062    Patent document 2: Japanese Laid-Open Patent Application Publication No. 2004-108162    Patent document 3: Japanese Laid-Open Patent Application Publication No. 2004-108163